Printed matters such as copied documents are obtained by various apparatuses, such as copying machines, printers and facsimile scanners. In order to evaluate the image-quality of these printed matters, it is necessary to measure various image-quality data related to image quality. Image-quality measuring devices have been used in order to measure these image-quality data.
When the above-mentioned measurements are carried out by using an image-quality measuring device, a printed matter is placed on a document platen as an original document, and illuminated by a illuminating device. The image of the original document is enlarged by using a microscope or other devices. The subsequent enlarged image is photographed by a CCD (Charge Coupled Device) camera or other devices so that an image signal is obtained.
A window, which serves as a display region having specific cells (pixels), is provided on a display screen such as a display of a computer, and an image derived from the image signal is displayed within the window. In accordance with the image signal, calculations are carried out on various evaluation items regarding image quality by using a computer or other devices so that various measured values are obtained as image-quality data. The image quality of the printed matter is evaluated by using these measured values.
The evaluation items include, for example, the degree of edge-irregularity of the line image, the degree of edge-blurredness of the line image, line-widths of the line image, etc.
However, upon carrying out the above-mentioned measurements, differences in magnification inevitably result in differences in resolution. For this reason, even in the case when the same image is measured, differences in magnification cause dispersion (irregularity) in the measured values. Consequently, in the case of measurements using different magnifications, such as in the case of different magnifications resulting from the application of different measuring devices and in the case in which different magnifications are used even in the same measuring device, it is not possible to simply compare the resulting measured values with each other. In other words, the resulting problem is that the measured values are not compared in an equivalent manner.
Moreover, in general, the image-quality data vary depending on the measuring magnification; and how they vary is affected by the image quality of the document, and it tends to differ. For this reason, even if a constant correction value is applied to the measured values regardless of the magnification in order to suppress the variation, it is not possible to obtain an appropriate correction in a specific magnification although it is possible to obtain an appropriate correction in other magnifications. Consequently, dispersion still occurs in the measurement precision.